1. Field of the Invention
This invention relates to devices for indicating the tensile load of an elongated support member such as a mine roof bolt. The invention is applicable to determining the tensile load of elongated bolts in general, and is particularly useful for determining the tension in a mine roof bolt, and the following discussion is directed to the device and its use in connection with mine roof bolting.
It is common practice in the preparation of a mine tunnel for mining operations to support the formation of rock above the mine tunnel roof with a series of mine roof bolts. Conventional practice involves drilling small diameter holes in the mine roof and inserting long anchor bolts into these holes. The bolts generally have an expansible anchor means on the end which is inserted into the drilled hole. The bolt head is rotated, causing expansion of the anchor against the interior of the drilled hole. Another type of roof bolt utilizes a settable resin injected around the bolt. Support is provided by the tensile load imparted to the bolt upon tightening the bolt head into tight abutment with the mine roof, generally through a mounting plate positioned between the mine roof and the bolt head.
It is desirable to know approximately the support provided by a given bolt, both at the time of installation and afterwards during mining operations.
2. Description of the Prior Art
The problem of determining the degree of support provided by mine roof support bolts has been appreciated in the prior art, and numerous devices of varying degrees of reliability have been developed. A common approach has been to provide a tension indicating device affixed to the lower end of the bolt to indicate the stress imparted to the bolt. U.S. Pat. Nos. 3,131,468; 3,161,174 and 3,169,440 are representative of this type of attached bolt tension indicators. A more sophisticated device comprising a load cell attached to the lower end of a mine roof support bolt is described in U.S. Pat. No. 3,872,719.
Another approach to the measurement of bolt tension involves inducing a vibration in the bolt and measuring reflected vibrations from the bolt as an indication of the stress in the bolt member. U.S. Pat. Nos. 3,307,393 and 3,918,294, and French Pat. No. 1,497,834 are representative of this approach to the problem. Additionally, U.S. Pat. No. 3,097,523 describes a sonic testing device for detecting flaws in metal members such as castings. The device described therein includes a striking hammer for applying a predetermined blow to the casting, and includes a microphone pickup and electronics to analyze the induced sound. Analysis of the induced sound compared to known characteristics induced by similarly striking a known good casting provides an indication of the quality of the struck casting.
While each of the devices discussed above has been useful in solving the problem of determination of support provided by mine roof bolts and the like, there has nevertheless been a continuing need for an instrument which would enable an operator to reliably determine the support provided by a mine roof bolt both at the time of installation and after the bolt has been installed for a period of time. Desirable characteristics of such a device include reliability, portability, and simplicity. The present invention provides these characteristics in a manner not heretofore available.